e borate of magnesia) give a clear
bead with soda, though it becomes slightly turbid by cooling when
saturated with magnesia, and crystallizes in large facets.

Magnesia and its compounds give beads with borax and microcosmic salt
similar to those of lime. By igniting magnesia or its compounds very
strongly in the oxidizing flame, moistening with nitrate of cobalt,
and re-igniting in the oxidation flame, they present, after a
continued blowing, a pale flesh-color, which is more visible when
cold. It is indispensable that the magnesia compounds should be
completely white and free of colored substances, or the color referred
to cannot be discerned. In general the reactions of magnesia before
the blowpipe are not sufficient, and it will be necessary to confirm
its presence or absence by aid of reagents applied in the wet way.

THIRD GROUP.--THE EARTHS, ALUMINA, GLUCINA, YTTRIA, THORINA, AND
ZIRCONIA.

The substances of this group are distinguished from the preceding by
their insolubility in water, in their pure or hydrated state--that
they have no alkaline reaction upon litmus paper, nor form salts with
carbonic acid. The earths are not volatile, and, in the pure state,
are infusible. They cannot be reduced to the metallic state before the
blowpipe. The organic salts are destroyed by ignition, while the
earths are left in the pure state, mixed with charcoal, from the
organic acids. The most of their neutral salts are insoluble in water;
the soluble neutral salts change blue litmus paper to red, and lose
their acids when ignited.

(_a._) _Alumina_ (Al^{2}O^{3}).--This earth is one of our most common
minerals. It occurs free in nature in many minerals, as sapphire,
etc.; or in combination with sulphuric acid, phosphoric acid, and
fluorine, and chiefly silicates. Pure alumina is a white crystalline
powder, or yellowish-white, and amorphous when produced by drying the
hydrate, separated chemically from its salts. Alumina is quite
unalterable in the fire; the hydrate, however, losing its water at a
low red heat. The neutral salts of alumina, with most acids, are
insoluble in water. Those soluble in it have an acid reaction upon
litmus paper, changing the blue into red.

The sulphates of alumina eliminate water when heated in a glass tube
closed at one end. By ignition, sulphurous acid (SO^{2}) is given off,
which can be recognized by its smell, and by its acid reaction upon
blue litmus paper, when a small strip of it mo